Fix QUICHE CI build.

This temporarily pins bazel to a pre-7.0 version, since Bazel 7.0 apppears to
break how we pull platform headers.

Also, this does not fix the broken tests yet, merely the compilation failures.

PiperOrigin-RevId: 594046756
diff --git a/.bazelversion b/.bazelversion
new file mode 100644
index 0000000..024b066
--- /dev/null
+++ b/.bazelversion
@@ -0,0 +1 @@
+6.2.1